1. Field of the Invention
This invention relates to a milling machine for machining crankshafts and similar workpieces, comprising milling units adapted to perform a feeding movement in a plane that is normal to the axis of the workpiece for machining the workpiece to form circular cross-sections which are concentric or eccentric to the axis of the workpiece, and a back rest for supporting the workpiece between the milling units in addition to the support of the workpiece at its ends, each of which milling units comprising a tool drum, adapted to be driven and to accommodate internally cutting annular inserted-tooth cutters.
2. Description of the Prior Art
Milling machines comprising two milling units and single or multiple inserted-tooth cutters as milling tools are used in the mass production of crankshafts or camshafts and similar workpieces and permit the workpieces to be machined within a short time because a plurality of cross-sections are machined at the same time. But in that case the workpiece is rather unstable and a machining to the desired accuracy will not be possible unless the workpiece which is gripped at its ends is additionally supported between its ends. Such an intermediate support of the workpiece gives rise to difficulties in internal milling operations, which are preferable to external milling operations because the edge life is longer and a better finish is obtained. But in internal milling operations the tool drums which surround the workpiece prevent the use of a back rest for supporting the workpiece between its ends. When it is desired to machine, e.g., all mainshaft portions or all crankpins of a crankshaft without a regripping of the workpiece, it has previously been necessary to provide for each milling unit a separate milling slide, which is mounted on an axially movable carriage, and to provide space for a back rest between the slides. In that case it is not possible to machine all mainshaft portions or crankpins at the same time but the machining must be performed in a plurality of steps, between which the milling units are longitudinally displaced. Besides, the slides required for the respective milling units must be provided with driving and control means operative in the direction of five or six coordinate axes, depending on whether the workpiece rotates or is at a standstill, and the axial displacement of the milling units involves a considerable increase of the nonproductive times so that the total machining time is increased; this is most undesirable.
In an attempt to utilize the advantages of internal milling in conjunction with short machining times, it has already been proposed in accordance with Laid-open German Application No. 31 17 030 to provide a milling machine with two machining stations, to machine one part of the crankshaft in one machining station and to machine the remainder of that crankshaft in the other machining station with a suitable time delay. In that case two crankshafts can be machined at the same time and each workpiece can be machined in each station at a plurality of cross-sections without a need for an axial displacement of the milling unit employed, and can be supported at the remaining portions, which are not machined at a given time, in addition to the gripping of the workpiece at its ends. But this concept involves high construction costs and requires each workpiece to be released in one station and regripped in the other station so that the nonproductive time is also increased. In addition the workpiece may be regripped in an eccentric position so that the machining may be inaccurate.